Approaches are being developed that use over the air power transmission between a transmitter and the device to be charged. These generally fall into two categories. One is based on the coupling of plane wave radiation (also called far-field radiation) between a transmit antenna and receive antenna on the device to be charged which collects the radiated power and rectifies it for charging the battery. Antennas are generally of resonant length in order to improve the coupling efficiency. This approach suffers from the fact that the power coupling falls off quickly with distance between the antennas. So charging over reasonable distances (e.g., >1-2 m) becomes difficult. Additionally, since the system radiates plane waves, unintentional radiation can interfere with other systems if not properly controlled through filtering.
Other approaches are based on inductive coupling between a transmit antenna embedded, for example, in a “charging” mat or surface and a receive antenna plus rectifying circuit embedded in the host device to be charged. This approach has the disadvantage that the spacing between transmit and receive antennas must be very close (e.g. mms). Though this approach does have the capability to simultaneously charge multiple devices in the same area, this area is typically small, hence the user must locate the devices to a specific area.
Due to variations in mutual inductances between a transmitting coil and receiving coils of different receivers plus further variations in mutual inductance seen at different locations within a transmitting coil, power delivery to the respective receivers can be markedly different during concurrent charging. It is desirable that a wireless power system to be able to direct power to the respective receivers depending on the power requirements and/or characteristics (bluetooth, phone, PDA) of the receivers, while at the same time maintaining the efficiency of the charging process to each and all of the receivers.
A need exists for methods, systems, and devices to for controlling power distribution among a plurality of receivers while maintaining power transfer efficiency to the plurality of receivers.